1. Field of the Invention
The present invention relates to heating furnaces of the elongated tunnel-type, and more particularly to such furnaces having a fixed lower section and a vertically retractable upper section.
2. Description of the Prior Art
Horizontal elongated tunnel-type furnaces are conventionally utilized for the heat treatment of various materials, such as glass, ceramics and metal, for example. The materials, or workpieces, oftentimes are in the form of plates or continuous strips and, generally, are carried through the furnace on a roller or belt-type conveyor. The furnaces are typically utilized to heat the workpieces in a controlled manner for further processing at a subsequent work station or stations.
Early heat treating furnaces generally comprised an elongated refractory chamber of rectangular cross section. The walls and roof were stationary members and included suitable heating means, such as gas and electrical resistance heaters, to heat the interior of the furnace.
The glass industry, particularly the flat glass industry, has realized considerable success with tunnel-type furnaces and has come to rely substantially on their use for the heat treatment of automotive and architectural glass. Glass sheets employed in the automotive industry, for example, are typically heated to their softening point while transiting a tunnel-type furnace, and then bent by suitable tooling to a prescribed configuration dictated by the design of the automobile in which they are to be installed After bending, the sheets which are to be employed as side lites and backlites are typically subjected to rapid chilling so as to develop a desired degree of temper. The sheets which are to be employed as windshields are gradually cooled so as to be annealed, and then laminated to another sheet or sheets in various combinations by means of a plastic interlayer.
Architectural glass is generally supplied in planar form and may be tempered, annealed and laminated much the same as automotive glass. The glass sheets undergo substantially the same processing treatment, except for the bending process, as described above for automotive glass. Of course, if the sheet is to be bent, bending is carried out prior to tempering, annealing and laminating.
Flat glass furnaces typically employ roll conveyor systems. The earlier conveyor systems generally comprised a series of spaced metallic rolls which extended across the furnace and were supported in journals located outside the side walls. A worm gear affixed to one end of each roll drivingly engaged a worm wheel on a drive shaft running along and mounted for rotation adjacent a side wall of the furnace. Such conveyer systems, while being mechanically sound, were relatively expensive to build and maintain, and did not readily admit to changing of individual rolls should that become necessary.
Rolls having ceramic glass contacting surfaces were found to have significant advantages over metallic rolls in the conveying of the sheets through the furnace. However, it was found that such rolls frequently require refurbishing to maintain their glass-contacting surface in a suitable condition, so that it is occasionally necessary to remove and replace individual rolls or groups of rolls. In order to facilitate changing of the rolls, so-called friction drive systems were devised wherein the rolls rest at each end, outside the furnace enclosure, upon longitudinally extending belts. The ends of the rolls are restrained against longitudinal movement so that as the flights of the belts upon which the rolls rest are advanced, the rolls are driven in rotation. Another roll drive system which provides ready access to rolls for servicing or replacement is disclosed and described in detail in copending application Ser. No. 07/485,643 filed Feb. 27, 1990. Changing of rolls and access to the furnace interior was further improved by the development of furnaces whose upper sections, that is, the portion above the roll line, comprises a vertically retractable assembly. A number of systems have been devised and implemented for raising and lowering the upper sections of such furnaces. While the systems have met with various degrees of success for their intended purpose, none has been found completely satisfactory from the standpoint of complexity, maintenance and overall costs.